Vocational vehicles, such as cement trucks, dump trucks, and farm tractors, are difficult to operate. In order to reduce operator fatigue and to allow operation by less experienced operators, it is desirable to equip such vehicles with automatic or partially-automated transmissions. It is also frequently necessary for such vehicles to operate at low vehicle speeds for extended periods. For example, when a cement truck is pouring cement along a curb, or a dump truck is spreading gravel along a roadway, or a farm tractor is planting crops, it is desirable for the vehicle to maintain a constant low speed, e.g., two to twelve miles per hour, regardless of the slope or type of terrain.
However, at such low speeds, the torque converter of the automatic transmission transfers only a fraction of the torque produced by the engine to the drive wheels, thereby reducing fuel efficiency with respect to manual transmission vehicles. Also, the resulting slip produces excess heat in the torque converter. Consequently, operation of such vehicles at low speeds for extended periods may reduce the operational life of the torque converter.
In addition, the engine output at low speeds is conventionally controlled by a torque governor. In order to maintain a constant low speed in a torque governing mode, the operator must alternately increase the throttle input in order to maintain the desired speed up slopes and apply the brakes in order to avoid exceeding the desired speed down slopes. Consequently, the operator must manipulate two inputs in order to govern the vehicle speed, thus increasing operator fatigue and taking the operator's attention away from other important tasks. Further, use of the service brakes to limit vehicle speed down slopes reduces the service life of the brakes.
U.S. Pat. No. 6,371,081 describes an engine controller having both a torque governor and a speed governor that may be simultaneously or selectively operated, based on the operator's manipulation of a torque throttle and a speed throttle. However, because the operator is required to manipulate both the torque throttle and the speed throttle in order to control the vehicle, the system described in the '081 patent increases the operator's work load. Moreover, the '081 patent makes no attempt to optimize control of the vehicle transmission in the torque governing and speed governing modes.
The presently disclosed systems and methods for controlling a powertrain are directed to solving one or more of these shortcomings of the prior art control systems.